1. Field of the Invention
This invention relates generally to a system and method for providing battery cell charge equalization and, more particularly, to a system and method for charging a fuel cell system battery on a vehicle during vehicle operation to provide battery cell state of charge equalization.
2. Discussion of the Related Art
Most fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental high voltage power source in addition to the fuel cell stack, such as a DC battery or an ultracapacitor. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor can provide regenerative braking that can also be used to recharge the battery through the DC bus line.
As is well understood in the art, a high voltage battery for this purpose typically includes several battery cells electrically coupled in series. As a result of many factors, such as internal resistance, electrical connections, battery aging, etc., the state of charge (SOC) of each cell in the battery may drift apart during operation of the battery over time. A battery management system may be coupled to the high voltage battery to monitor the state of charge of each battery cell and the temperature of the battery, and control how much the battery can be charged and discharged based on the state of charge of the maximum charged cell and the minimum charged cell. Particularly, the battery cannot be used to provide power if the cell with the lowest state of charge will drop below some minimum state of charge because that cell may become damaged, and the battery cannot be over charged beyond some maximum state of charge for the cell with the highest state of charge because that cell may become overheated and damaged. Thus, a cell with a low state of charge may prevent the battery from being used even though the other cells may have a suitable or significant state of charge.
Currently, the only remedy to equalize the state of charge of the cells of a battery is to take the vehicle to a service center and charge the battery with a special charging device that provides cell overcharging while closely monitoring the temperature of the battery so that all of the cells become fully charged and are equalized. Clearly such a solution has obvious drawbacks because the vehicle cannot be driven during such service and includes the obvious inconvenience of having to take the car to the service center.